In a multiple-ratio automatic transmission of the kind shown, for example, in U.S. Pat. No. 4,938,097, a multiple-ratio transmission is situated on the torque output side of the hydrokinetic torque converter, the impeller of the converter being connected through the driveline damper to the crankshaft of an internal combustion engine. The turbine of the torque converter is connected to a torque input element of the gearing through a selectively engageable forward-drive friction clutch during operation in forward drive. A separate reverse drive friction clutch connects the turbine of the torque converter to another torque input element of the gearing during operation in reverse.
The torque converter of such known transmissions occupies a substantial space in the driveline assembly between the transmission gearing and the engine. It also is of a relatively large diameter relative to the diameter of the transmission gearing. Further, the converter has a substantial rotating mass, which must be accelerated by the engine during startup of the vehicle in forward drive or in reverse drive.
The effective mass of the converter, of necessity, includes the mass of the hydraulic fluid that circulates in the torus circuit defined by the converter impeller, the turbine and the stator.
The operating efficiency of the converter during startup is low. It varies from a zero value at stall to a maximum value of approximately 80-85% at the coupling point. The coupling point occurs at the transition from the torque multiplication mode to the coupling mode where the torque multiplication ratio is unity. The converter inefficiency results in power loss, which is manifested by an increase in temperature of the hydrokinetic fluid. This necessitates the use of oil coolers, which further add to the gross weight of the transmission assembly and which require additional packaging space for the automotive vehicle driveline.
The torque converter functions, as indicated above, to provide a smooth launch of the vehicle from a standing start. It also disconnects the engine from the driveline when the vehicle is at rest. A third principal function is the dampening of engine torsional vibrations.
Although the torque converter achieves a high torque multiplication ratio of about 2:1 at the outset of a vehicle launch, this torque ratio is accompanied by acceleration of a relatively large mass, which reduces useful torque at the vehicle wheels. The same is true for a transmission of the type disclosed in U.S. Pat. No. 5,836,849, which describes a large diameter startup clutch between an engine and a transmission torque input shaft.